The leveling curve becomes flat after the first 10 levels. After level 10, leveling up will take about 1h to 1h30m. This doesn’t change as you level up.

I am a human thief and currently lvl 15 - I have completed every part of my storyline (next one is for a lvl 17) and I have all but one quest hub completed (which is a lvl 17 suggested). I have all vistas explored and have completed about 20 of the group quests while playing. I also have 100% of Divinity’s Reach explored. And I’m lvl 21 and 29 in two professions.But I am stuck at 15 and cannot find a reasonable way to level up. 8exp per mob is not cool. Is there something I'm not doing correctly? I've played WoW for years, Rift for months, and even Warhammer but have never had this issue with lack of quests.

Am I missing some kind of huge exp giver? Should I just run around doing the group quests over and over?

There are zones beyond Queensdale, and you don't need to wait for the story (or anything really) to go visit them. Check the fort south of the swamp, the SE corner, or the NW corner for portals to higher level zones.

Queensdale is rated for levels 1-15, no surprise that you are done at 15.

On the other hand, you can always farm XP repetitively anywhere if you find a good chain of events to follow.

I moved on to another starting zone. I 100% completed Caledon Forest/Grove exploration for Sylvari and have now moved on to Metric Provence for Asura, while doing DE's, gathering materials and crafting along the way and I'm level 18.